Particle accelerating tube having axially localised transverse magnetic fields and field-free regions

ABSTRACT

1,116,442. Particle accelerators. UNITED KINGDOM ATOMIC ENERGY AUTHORITY 16 Nov., 1965 [19 Nov., 1965], No. 47260/64. Heading H1D. In a D.C. particle accelerator at least one &#34; quartet&#34; of axially localized transverse magnetic fields is provided along the acceleration tube to deflect the beam successively away from the axis of the tube, parallel to the axis, towards the axis and then along the axis again or, in practice, along a path parallel to, but slightly displaced from the axis, as shown. Spurious electrons are, however, deflected into the electrodes and the electron loading is thus reduced. The magnetic fields may be set up by one or more pairs of permanent magnets or by annular permanent magnets incorporated in selected electrodes, i.e. mounted in the dished portions of selected electrodes with coverplates provided to shield the magnets from the electric fields. At the input end of the acceleration tube electrodes with circumferentially corrugated apertures may be used to provide non-axial electric fields. Alternatively electrodes with non-corrugated circular apertures progressively decreasing in diameter may be used. In an arrangement with pairs of permanent magnets the apertures of the electrodes have enlarged end portions to aid evacuation and successive electrodes are rotated, in certain regions where the magnetic fields are closely spaced by 90 degrees, to provide optical baffling. In a second arrangement with annular magnets Fig. 7 (not shown), relatively large apertures are used to increase pumping speed. Electrodes dished in either direction relative to the beam may be used. In the arrangement shown, certain of the electrodes may be flat with a transitional degree of dishing of the electrodes on either side in order to avoid any reduction in the number of electrodes along the acceleration tube due to the incorporation of the magnets. A progressive dishing of the electrodes adjacent those carrying the magnets may also be used in the second arrangement, Fig. 7 (not shown). In the arrangement shown, successive &#34; quartets &#34; divert the beam to opposite sides of the tube axis. In the second arrangement, Fig. 7 (not shown), the beam is always diverted to the same side. The accelerator may be for accelerating ions or electrons.

Sept. 17, 1968 F. A. HOWE ETAL 3, PARTICLE ACCELERATING TUBE HAVING AXIALLY LOCALISED TRANSVERSE MAGNETIC FIELDS AND FIELD-FREE REGIONS Filed NOV. 17, 1965 5 ShGQtS-Sh'Ot 1 3 r r r ig s 5&2Z2;If??? 1 a m w vi; r rv Q fi m? Tff .f a L 2. h a x r r F L W E E QQN Sept-17, 1968 F A. HOWE ETAL 3,402,310

PARTICLE ACCELERATING TUBE HAVING AXIALLY LOCALISED TRANSVERSE MAGNETIC FIELDS AND FIELD-FREE REGIONS Filed Nov. 17, 1965 5 Sheets-Sheet 2 FIG.3.

III/III IIIIIIIIJAI Sept. 17, 1968 F. A. HOWE ETAL 3,402,310

PARTICLE ACCELERATING TUBE HAVING AXIALLY LOCALISED TRANSVERSE MAGNETIC-FIELDS AND FIELD-FREE REGIONS Filed Nov. 17, 1965 5 Sheets-Sheet 5 ELECIRUIJE MIMBER l SIXV HUM INBHSJV'HSIII "V38 Sept. 17, 1968 F. A. HOWE ETAL 3,402,310

PARTICLE ACCELERATING TUBE HAVING AXIALLY LOCALISED TRANSVERSE MAGNETIC FIELDS AND FIELD-FREE REGIONS Filed Nov. 17, 1965 5 Sheets-Sheet 4 FIGB.

BEAM DISPLACEMENT FM AXIS lmml Sept. 17, 1968 F. A. HOWE ETAL PARTICLE ACCELERATING TUBE HAVING AXIALLY LOCALISED TRANSVERSE MAGNETIC FIELDS AND FIELD-FREE REGIONS 5 Sheets-Sheet 5 Filed NOV. 17, 1965 Q? M, a m p q g MW NUE United States Patent 3,402,310 PARTICLE ACCELERATING TUBE HAVING AXIALLY LOCALISED TRANSVERSE MAG- NETIC FIELDS AND FIELD-FREE REGIONS Frederick Albert Howe, Newbury, and Ronald Inch Bell, Basingstoke, England, assignors to United Kingdom Atomic Energy Authority, London, England Filed Nov. 17, 1965, Ser. No. 508,242 Claims priority, application Great Britain, Nov. 19, 1964, 47,250/ 64 7 Claims. (Cl. 313-63) ABSTRACT OF THE DISCLOSURE A linear particle accelerating tube including an arrangement for deflecting electrons to minimize the phenomenon known as electron loading. The arrangement includes a plurality of spaced apertured electrodes and at least two pairs of devices for producing axially localized transverse magnetic fields separated from one another by substantially field-free regions, each successive pair of localized fields being oriented to deflect the accelerated particle beam from one selected path substantially parallel to the tube axis to another selected path substantially parallel to the tube axis. The paths are selected to minimize the net displacement from the tube axis of the beam emerging from the tube while deflecting the unwanted electrons into the electrodes.

This invention relates to linear particle accelerating tubes suitable for use with electrostatic accelerators, e.g. of the Van de Graafl type.

Such tubes commonly consist of a large number of accelerating electrodes in the form of circular plates or dishes, sealed to and separated by annular glass insulators. The electrodes having aligned holes through which passes the ion beam. The performance of an electrostatic generator as a particle accelerator is normally limited by the quality of the accelerating tube, which limits the maximum accelerating voltage which can be achieved. In particular the phenomenon known as electron loading occurs in which unwanted electrons, e.g. produced by field emission from the electrodes, are accelerated back towards the positive end of the tube, generating X-rays as they strike the accelerating electrodes, the gap lens preceding the tube, or the ion-source assembly. These X-rays ionise the highpressure gas surrounding the tube and so reduce the eflective insulation, while the electrons themselves constitute a load on the generator, tending to reduce its output voltage.

It has been proposed to reduce the electron-loading effect by tilting the accelerating electrodes at an angle to the tube axis in such a Way as to produce electric fields which deflect the electrons to strike the electrodes before vthey are accelerated to energies at which X-ray production becomes efficient. An arrangement of this kind is described, for example, in UK. specification No. 967,963. The present invention provides an alternative arrangement directed towards the same purpose.

According to the present invention a linear particle accelerating tube comprising a plurality of spaced, apertured electrodes includes means for producing axially localised transverse magnetic fields spaced along and with the tube and separated by substantially field-free regions, said fields being adapted to deflect unwanted electrons into the electrodes but to allow an accelerated particle beam to leave the tube on a path substantially parallel to the tube axis.

Also according to the present invention a linear particle accelerating tube comprising a plurality of spaced, apertured electrodes includes at least one quartet of means, spaced along and within the tube, for producing axially localised transverse magnetic fields, said fields being sepaice rated from one another by substantially field-free regions and aligned to deflect a charged particle beam in a common longitudinal diametral plane, the two outer means of the quartet being oriented to deflect the beam in one diametral direction and the two inner means of the quartet being oriented to deflect the beam in the opposite diametral direction.

Each said means preferably comprises a magnetic polepair mounted on a said electrode, and said pole-pairs are preferably constituted by permanent magnets.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings wherein FIGURE 1 is a diagrammatic longitudinal section of an accelerator tube embodying the present invention.

FIGURES 2 and 3 are a plan View and cross-sectional elevation respectively of a dished electrode fitted with a magnetic pole-pair.

FIGURE 4 is a graph showing changes in the position of a beam as it traverses the tube.

FIGURES 5 and 6 are views corresponding to FIG- URES 2 and 3 of a further embodiment of the invention.

FIGURE 7 is a view corresponding to FIGURE 1 of this further embodiment.

FIGURE 8 is a graph corresponding to FIGURE 4 for this further embodiment.

FIGURE 1 shows a plurality of electrodes forming an accelerator tube and numbered 1 to 142 from the input end of the tube. The electrodes are spaced apart by annular glass insulators I in the usual way. The tube employs three quartets of permanent-magnet pole-pairs; the first quartet is fitted in dished electrodes 31, 36, 48 and 54; the second in electrodes 65, 74, 84 and 94; and the third in electrodes 104, 114, 124 and 135. The two poles of each pole-pair are marked N (north) and S (South) respectively. It will be seen that the two outer pole-pairs of each quartet, viz. at electrodes 31 and 54, 65 and 94, 104 and 135, have their poles oriented in the opposite direction from the inner pole-pairs of their respective quartets, viz. at electrodes 36 and 48, 74 and 84, 114 and 124.

FIGURES 2 and 3 show one of the above-numbered electrodes in more detail, comprising a pair of permanent magnets having their respective N and S poles oriented as shown, fitted within the dished portion of an aluminium electrode E and joined by a circumferential yoke Y. The ion beam passes between the poles through an aperture A in the electrode. The end-portions of the aperture are enlarged to form lobes B which aid evacuation of the tube, and the poles and yoke are covered by an aluminium plate C having an aperture of the same dimensions as that in the electrode. Cover plate C which, like electrode E, has rounded edges and is highly polished, is provided to shield the magnets and the steel yoke from the strong electric field to which the electrode is subjected in use.

FIGURES 2 and 3 are approximately to scale, the width of the aperture A in the present embodiment being 2 inches. The sintered Ticonal G permanent magnets are 1 inch deep in the axial direction and provide a field of about 400 gauss in the aperture. The electrodes are spaced 1.1 inches apart by the annular insulators.

The pole-pairs are arranged to produce transverse magnetic fields which deflect from the aperture region, into the electrodes, any electrons tending to travel back towards the input end, without producing any substantial net deviation of the beam from its axial path. Referring again to FIGURE 1, the path of the ion beam is shown by the solid line D. Assuming the beam path to be axial on reaching the first pole-pair of the third quartet, at electrode 104, the localised field produced by this polepair deflects the beam through an angle or towards one side of the tube. (Although FIGURE 1 shows this deflection to be in the plane of the paper, it actually occurs,

of course, in a plane normal to that of the paper.) The beam continues in the deflected direction until it reaches the pole-pair in electrode 114 which, being oppositely oriented, deflects the beam back through the angle a so that its path becomes parallel to the tube axis but displaced fro-m it. Similarly at electrode 124 the beam is deflected back towards the axis, and at electrode 135 resumes a substantially axial path. (In fact, as shown in FIGURE 4, the beam resumes a path parallel to, but slightly displaced from the axis.)

Although the deflected path is shown as linear, making an angle a with the axis, it is in fact parabolic because of the increasing energy of the beam. Hence, for example, the angle a at which the beam reaches electrode 114 is in fact slightly less than the angle a at which it left electrode 104. This effect is compensated for, however, by the fact that the deflection applied at electrode 114 is similarly reduced because of the increased energy, so that the beam leaving electrode 114 continues substantially parallel to the axis.

The first and second quartets behave similarly. It will be seen, however, that although the pole-pairs of the second and third quartets are approximately equispaced along the tube, those of the first quartet are arranged as two consecutive pairs relatively close together. This is because at the input end of the tube the beam energy is relatively low and hence the beam is very sensitive to deflecting fields. To prevent the beam being deflected too far from the axis, the pole-pairs (at electrodes 36 and 54) which restore the beam to its parallel and axial paths are therefore placed relative close to the deflecting pole-pairs 31 and 48 respectively.

FIGURE 4 shows the calculated displacements about the tube axis for a proton beam entering the abovedescribed tube on an axial path at an energy of kev. and leaving at 2 mev. It will be seen that the spacing between the adjacent electrodes carrying pole-pairs is such as to give the path of the emerging beam the very small net displacement of 0.064 mm. A uniform electric field of 15 kv. between adjacent electrodes 1-135 was assumed in this calculation, the remaining electrodes not being used for accelerating. In the present embodiment the successive quartets deflect the beam to opposite sides of the axis. This is not essential, and an alternative arrangement is shown in FIGURE 7.

For electrons emitted by the electrodes and travelling towards the input end of the tube, the deflection produced by each pole-pair is much greater (owing to their small mass relative to the ions), and these are deflected into the electrodes before they have attained energies at which X-ray generation becomes efficient. In the present tube, which is intended to accelerate ions to energies in the range 1.5 to 6 mev., the pole-pairs are not more than approximately ten electrodes apart, corresponding to a maximum electron energy before deflection by a polepair of about 400 kev.

It Will be seen that at the input end of the tube, electrons leaving electrode 31 could apparently reach the 1011 source with an energy of about 1.2 mev. (with a total applied voltage of 6 mv.) without being deflected. However, for the reason already explained, namely the sensitivity of the ion beam at low energies (especially relevant when the total applied voltage is reduced to 1.5 mv.), magnetic deflection of the electrons is not used in this region in the present embodiment. A corresponding degree of electron loading can be tolerated, or alternative electron-suppression systems employed, such as the use of electrodes having circumferentially corrugated apertures to provide non-axial electric fields, as described in cO-pending US. application Ser. No. 481,593, filed Aug. 23, 1965, now Patent No. 3,363,125 issued Jan. 9, 1968. In the present embodiment electrodes 1-30 have conventional non-corrugated circular apertures whose diameters decrease progressively as shown.

The electron-deflecting fields provided by the polepairs are effective to suppress electrons within the aperture A (FIGURE 2). In the two lobes B only a fringe magnetic field exists, and to prevent electrons being accelerated along the tube through these lobes, successive electrodes between those carrying pole-pairs (except those immediately adjacent the latter) are rotated progressively through an angle relative to the preceding electrode so that the total rotation between adjacent electrodes carrying pole-pairs is 180. The projecting portions F of the rotated electrodes form an helix along the tube, so that no unobstructed optical path parallel to the axis exists through the lobes B between adjacent electrodes carrying pole-pairs. This method (optical bafliing by successive electrode rotation) of electron suppression in a tube having lobed apertures of the described form to give eflicient evacuation can be applied without magnetic suppression, and may be sufficient in tubes only required to operate at lower voltages.

Electrodes 31 and 36, and 48 and 54, are too close together for the above method to be adopted. In these portions of the tube electrodes 33 and 34, and 50, 51 and 52, are rotated through so that the portions F of these electrodes are in register with the lobes of the adjacent electrodes. This arrangement provides optical bafiling without significant increase of pumping impedance.

It will be seen that in each dished electrode carrying a pole-pair, the cover plate C occupies approximately the plane which a preceding dished electrode would occupy in a conventional tube. Thus, if similarly dished electrodes were used throughout, the number of electrodes in the present tube, for a given total length, would have to be reduced, as compared with a conventional tube without pole-pairs, by the number of pole-pairs fitted. Thus for a given permissible voltage stress per insulator, the maximum applied voltage would have to be reduced. In the present embodiment the number of electrodes (and hence insulators) is maintained at the conventional value by progressively reducing the degree of dishing in some of the electrodes between each pole-pair. For example starting from the pole-carrying electrode 124, electrodes -128 are fully dished, electrodes 129-133 are progressively less dished, and electrode 134, which precedes the next pole-carrying electrode 135, is not dished at all. The use of dished electrodes to screen the insulators in an accelerating tube is desribale but is not essential, particularly as the beam energy increases.

It will be apparent that the first and second pole-pairs of each quartet, e.g. in the third quartet those in electrodes 104 and 114, should produce equal-strength fields in order to return the beam to a path parallel to the tube axis, and similarly that the third and fourth pole-pairs, i.e. those in electrodes 124 and 135, should produce equalstrength fields. It is not essential, however, that the fields produced by the third and fourth pole-pairs should be the same as those provided by the first and second pole-pairs The above-described tube has been used at up to 6 MV without measurable electron loading.

Instead of using a single magnetic pole-pair to produce each localised transverse magnetic field, the poles may be distributed over two or more adjacent electrodes. The latter arrangement has the advantage that, to produce a given deflection angle, the individual magnets can be weaker (since the field is applied over a greater length of tube), and hence the magnets can be shallower in the axial direction, making it feasible to use similarly dished electrodes throughout.

Instead of using separate magnets linked by a yoke, ring-type magnets can be used, magnetised across a diameter. FIGURES 5 and 6 show a ring magnet M fitted in a dished electrode E and protected by a cover-plate C, the assembly being held together by two rivets R which pass through holes in the North and South polar regions of the magnet, and FIGURE 7 shows an accelerator tube comprising eight such electrode assemblies forming two quartets. In this tube, designed to operate at up to 3 mev.,

each magnet M is of Ticonal G and produces a field of about 100 gauss. The aperture A (FIGURE 6) is 3.5 inches in diameter and the magnet is 0.375 inch deep in the axial direction.

In FIGURE 7 the first quartet of pole-pairs is fitted in electrodes 17, 23, 30 and 38; the second in electrodes 44, 50, 57 and 64. Unlike the embodiment of FIGURE 1, the pole-pairs of the two successive quartets (marked N and S) are oriented to deflect electrons to the same side of the tube axis. The arrangement has the advantage that, apart from the first and last pole-pairs (those fitted in electrodes 17 and 64), the electrons meet successive pairs of deflecting fields oriented in the same direction, which ensures that any electrons insufliciently deflected by the first field are further deflected by the second. This is particularly useful where smaller fields are used, as in the present embodiment. FIGURE 8 shows the calculated displacements about the tube axis for a proton beam entering at any energy of 3 kev. and leaving at 210 kev. The net displacement is approximately 0.125 mm. and is parallel to the tube axis. The proton beam deflection clearly increases as the voltage gradient applied to the tube is reduced. Hence FIGURE 8 is plotted for an applied gradient of 3 kv. between adjacent electrodes, which is the lowest gradient likely to be used in practice.

In FIGURE 7 it will be seen that the electrodes are dished in the opposite direction, relative to the directions of motion of the protons and electrons, to the electrodes of FIGURE 1. The direction shown in FIGURE 7 is more effective in causing the deflected electrons to strike succeeding electrodes instead of bombarding the inner surfaces of the glass insulators I.

With the exceptions noted below, the electrodes in FIG- URE 7 are dished to a standard depth of 1.1 inch, the insulators I separating them by 0.9 inch. As in the embodiment of FIGURE 1, the inclusion of ring magnets in eight of the electrodes tends to reduce the spacing between these and the adjacent electrodes. In the present embodi ment this difliculty is overcome by reducing the dishing of the two electrodes adjaecnt to each magnet-carrying electrode on the concave side thereof. Each magnet-carrying electrode (e.g. electrode 17) is dished to the standard depth of 1.1 inch, but 0.46 inch of this is taken up by the magnet and cover plate (which, like the electrode, is 0.085 inch thick), leaving 0.7 inch. To compensate for this, the next adjacent electrode (e.g. electrode 18) is dished to only 0.7 inch and the next electrode to that (e.g. electrode 19) is dished to only 0.9 inch. As a result the effective spacing between electrodes 17 and 18, 18 and 19, and 19 and 20 is not reduced to less than 0.7 inch. The same arrangement is used adjacent to the seven other magnet-carrying electrodes.

Unlike the embodiment of FIGURE 1, the apertures A in the present embodiment are not lobed. The apertures of the eight magnet-carrying electrodes are 3.5 inches in diameter. In order to obtain maximum pumping speed the apertures of most of the remaining electrodes are enlarged to 4.375 inches. The exceptions are the final electrodes, 69 and 70, which are 3.5 inches in diameter, and the initial electrodes which provide a tapered input in a known manner. Of the latter, electrode 2 has an aperture diameter of 3 inches; electrodes 3 to 7 have 4.375 inch apertures; electrodes 8 to 13 have 4.25 inch apertures; and electrodes 14 to 16 have 4.125 inch apertures.

Although described with reference to the acceleration of beams of ions, the invention can also be applied to tubes for accelerating electrons, since the transverse fields will produce much smaller deflection of the high-energy electron beam than of unwanted locally-emitted electrons.

We claim:

1. A linear particle accelerating tube comprising a plurality of spaced, apertured electrodes and including at least one quartet of means, spaced along and within the tube, for producing axially localised transverse magnetic fields, said fields being separated from one another by substantially field-free regions and aligned to deflect a charged particle beam into paths parallel to a longitudinal plane, said plane being a common diametral plane, the two outer means of the quartet being oriented to deflect the beam in one diametral direction and the two inner means of the quartet being oriented to deflect the beam in the opposite diametral direction.

2. A tube as claimed in claim 1 wherein each said means comprises a magnetic pole-pair mounted on a said electrode.

3. A tube as claimed in claim 2 wherein said magnetic pole-pairs are constituted by permanent magnets.

4. A tube as claimed in claim 3 wherein said permanent magnets are mounted within dished portions of said electrodes.

5. A tube as claimed in claim 4 wherein said permanent magnets are ring magnets magnetised across a diameter.

6. A tube as claimed in claim 1 wherein successive quartets .are oriented to deflect electrons to the same side of the tube axis.

7. A linear particle accelerating tube comprising a plurality of spaced, apertured electrodes and including at least two pairs of means spaced along and within the tube for producing axially localized transverse magnetic fields separated from one another by substantially fieldfree regions, said localised fields being aligned to deflect a charged particle beam into paths parallel to a common diametral longitudinal plane, the first said means of each pair being oriented to deflect the beam in one diametral direction and the second said means of each pair being oriented to deflect the beam in the opposite direction, each pair thereby being oriented to deflect the beam in the 0pposite direction, each pair thereby being operative to deflect the accelerated particle beam from one selected path substantially parallel to the tube axis to another selected path substantially parallel to the tube axis, said paths being selected to minimize the net displacement from the tube axis of the beam emerging from the tube, while deflecting unwanted electrons into the electrodes.

JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner. 

